Improve compressed image workflows

[greeble-dev]

What problem does this solve or what need does it fill?

#23567 and #24223 improved the compressed image workflow by adding BCn/ASTC support, mipmapping, and other features. However, there's some problems:

1. Targeting multiple platforms is awkward.
   • Users may want to choose between BasisU/BCn/ASTC per target platform.
   • But BasisU and BCn/ASTC are mutually exclusive at compile time (see compressed_image_saver and compressed_image_saver_universal features).
   • And choosing between BCn/ASTC is an environment variable.
   • Both of these can be a problem if a single CLI or editor process/binary wants to handle multiple platforms.
2. The default processing behavior is inflexible.
   • ImagePlugin:::build registers a default processor that automatically compresses *.jpg, *.png.
   • Maybe the user doesn't want this by default.
   • Or maybe they want other file extensions (.tga etc).
3. Embedded images are not supported.
   • There's no workflow for images embedded in glTF files.

What solution would you like?

1. Targeting multiple platforms is awkward.
   • The asset processor should have general support for target platforms - this is not specific to images (see Bevy Asset V2 Tracking Issue #9714 under "Per-Platform Processed Assets").
   • Assuming that general API is added, the compressed image saver might just need some light restructuring.
2. The default processing behavior is inflexible.
   • Adding settings to ImagePlugin seems like the obvious solution, at least for the short-term.
3. Embedded images are not supported.
   • Open question, and arguably a general asset processing issue.
   • Also depends on if/how the glTF loader ends up working with BSN.
   • Maybe this gets solved as a side effect of other pipeline changes or processor improvements.

What alternative(s) have you considered?

2. The default processing behavior is inflexible.
   • Users can register their own default processor for other file extensions (but can't remove the one ImagePlugin adds).
   • Users can add .meta files to override the default processor.
3. Embedded images are not supported.
   • For glTFs, users can compress the images themselves or resave the glTF with non-embedded images.

[greeble-dev]

#24904 addresses (or partly addresses?) the "default processing behavior is inflexible" issue.